Variable gain amplifiers find widespread use in wireless transceivers. They are used in receivers to compensate for varying input levels and in transmitters to adjust the output power level.
A typical receiver includes a switched-gain low noise amplifier (LNA) and multiple baseband variable gain amplifiers. The switched-gain LNA provides at least two modes—high gain and bypass. It invariably suffers from switching transients that adversely affect the magnitude and phase of the received signal, which can be avoided with an LNA offering continuous gain control.
The receiver is generally characterized by its sensitivity and selectivity. The sensitivity of the radio receiver measures the minimum signal that can be detected and demodulated. This takes into account the noise figure of the radio receiver FT, the bandwidth B of the system, and the performance of the demodulator.
The noise figure of a cascaded radio receiver depends on the gain and noise contributed by each circuit, with the first few stages dominating. The selectivity, or linearity, of the radio receiver indicates the largest interfering signal that can be rejected by the system. These signals create intermodulation distortion products that increase rapidly as the interfering signal level increases and degrade receiver signal quality. As a result, the later stages in the receiver system dictate overall linearity.
The sensitivity and selectivity of the radio receiver create conflicting requirements. It therefore becomes advantageous to adapt the receiver to the operating environment and this requires individual control of the LNA and all the variable gain amplifiers.
A transmitter similarly uses baseband and radio frequency (rf) variable gain amplifiers to set the output power level, which varies dramatically in some systems such as in code division multiple access (CDMA) systems.
The transmitter is generally characterized by its maximum output power, distortion, and efficiency. The maximum output power relates to the range of the radio transmitter. It's limited by distortion, which causes spectral regrowth and interferes with nearby radio channels. In turn, amplifier distortion depends on the input signal amplitude and the operating bias, which ties directly to the efficiency of the system.
The gain of the different variable gain amplifiers directly affects the performance of the transmitter. As such, it becomes advantageous to adjust the transmitter based on its output power level and this requires control of each variable gain amplifier.
An automatic gain control (AGC) network must provide monotonic gain control with some precision; otherwise, problems develop due to feedback. Ideally, the AGC network also displays a linear-dB control response. The resulting linear control signal varies exponentially and greatly magnifies errors plus discontinuities due to segmenting of the gain control response. It would therefore be advantageous to have an AGC network that operates to overcome the above problems.